The invention relates to a method for damping electromechanical oscillations in an electromechanical system, particularly for damping electromechanical oscillations in an automotive electric or hybrid drive system such as for instance a hybrid vehicle, and an oscillation damping system for employing such method.
Electromechanical systems are frequently prone to exhibit torsional resonances that may cause torsional oscillations. A main cause is commonly mechanical spring-dampers that should mitigate higher frequency torque alternations to propagate throughout the system. However, in conjunction with an electric machine that is controlled by an electrical device connected to an electrical system, the mechanical system may interfere with the electrical system and the controls thereof. This can abate mechanical system damping properties. In order to mitigate such oscillations, it is common to apply damping control by utilizing the control device of the electric machine.
Oscillations in the drive system of a vehicle can have a detrimental effect on the vehicle.
In hybrid vehicles, for instance, comprising a combustion engine and an electric machine, the combustion engine as well as the electric machine can serve as drive sources. When switching from a purely electric drive mode to a drive mode with the combustion engine, the engine engagement invokes severe oscillations due to engine friction before the engine fires and the surge after the engine fires.
These oscillations are transmitted through the driveline and are ultimately felt by the occupants of the vehicle.
Oscillation control systems are employed for damping such or similar oscillations.
For instance, U.S. Pat. No. 5,537,967 discloses an oscillation damping system for a vehicle including a torque generator with given operation timing to produce a torque oscillation of a preselected frequency and an operation timing controller. The operation timing controller initially determines a phase difference between the engine revolution and the frequency of the vehicle oscillation formed of a resultant vector defined by a first oscillation component by movement of the crankshaft of the engine and a second oscillation component caused by movement of a piston of the engine. The controller then modifies the operation timing of the torque generator to provide the torque oscillation in an opposite phase relative to the vehicle oscillation based on the phase difference between these two oscillations and determined to compensate the vehicle oscillation.
GB 2346351 discloses a motor vehicle with a combustion engine comprising an electric machine which is used for actively damping oscillations due to torque changes caused by the combustion engine and by an engagement of the clutch.
It is desirable to provide an improved method for damping electromechanical oscillations in an electromechanical system which can be employed for electric machines operated in a saturated state, particularly for damping electromechanical oscillations in an electric or hybrid vehicle. It is also desirable to provide an improved oscillation damping system for damping electromechanical oscillations in such an electromechanical system.
A method according to an aspect of the invention is proposed for damping electromechanical oscillations in an electromechanical system, particularly in an automotive electric or hybrid drive system, comprising at least one electric machine operable with an angular speed and a phase of said angular speed, being coupled to at least one torque load and producing an electromotive force, comprising the steps of deriving actual values of the electromotive force; adjusting the phase of the angular speed, based on the derived actual values of the electromotive force, in a direction in which an oscillatory behaviour of said electric machine is reduced.
Typically, the unwanted electromechanical oscillations have a major impact mainly on the mechanical system connected to the electrical drive system.
The electromotive force is derived from the magnetic flux and the electrical angular speed of the electric machine and is a term used to characterize electrical devices, such as e.g. electrical generators. For a given device, if an electric charge passes through that device, and gains energy, the net electromotive force for that device is the energy gained per unit charge. For an electric machine, the source of electromotive force is electromagnetic induction. Because the electrical angular speed is a relation between the pole number of the electric machine and the mechanical angular speed, any oscillation apparent in the mechanical angular speed can also be observed in the electromotive force. The electromotive force can be calculated from the electrical angular speed and the pole number or can be measured or estimated via a state observer in the system.
The invention utilizes internal properties of the electric machine current control for altering the resonant electromechanical system. Favourably, the electromechanical oscillations in an electromechanical system can be damped. The system may comprise at least one controllable electric machine and at least one torque load which is connected to the electric machine by a mechanical spring-damper system which may have damping properties.
The invention is particularly useful for a control of an electric machine being operated in saturation due to torque limitation and/or voltage limitation. More particularly, the invention relates to commonly un-damped or poorly damped inherent electromechanical resonances encompassing a control unit comprising a torque control and current control of the electric machine and a mechanical system which comprises at least two systems of inertia interconnected by a spring system which may exhibit damping properties. In this sense, the electric machine consists of or comprises an electrical system describing the electrical properties, particularly an electric circuit, and a mechanical system of inertia describing the inertia of the electric machine. The other system of inertia may comprise an engine and/or other kind of torque loads such as wheels, a transmission etc.
The coupling between the control system of the electric machine, particularly the current control system, and the mechanical system, i.e. the systems of inertia and the mechanical coupling between these systems of inertia, creates a resonant system. Other than in prior art solutions which commonly address deviations in electrical and mechanical angular speeds, respectively, between the mechanically connected systems of inertia and which calculate an additional torque component which is added to the reference torque for improving the damping properties, the method according to the present invention changes an inherent coupling of the systems of inertia in a way that mitigates the oscillatory behaviour of the resonant system.
According to a favourable method step, the step of deriving the actual values of the electromotive force can comprise at least one of the steps:                deriving said values from the calculated magnetic flux of said electric machine;        deriving said values from the calculated electrical angular speed of said electric machine; and        deriving said values from an estimation made by an observer of the state of the electric machine.        
Particularly, the electrical angular speed can be calculated from values of a mechanical angular speed of said electric machine and a pole pair number of said electric machine. Such parameters are easily accessed in the electromechanical system comprising the electric machine.
According to a favourable method step, the step of deriving actual values of the electromotive force can be conducted in parallel to and independent from at least one of the further steps (i) of controlling a torque of the electric machine and (ii) of controlling any torque request. Favourably, limitations e.g. of current or torque which are valid for, and have to be observed by, this torque control segment have no impact on the performance of the method according to the invention.
According to a favourable method step, at least one of the steps (i) deriving actual values of the electromotive force and (ii) adjusting the phase of the angular speed can be performed by using a filter, in particular by using the filter for processing the step of deriving actual values of the electromotive force in form of a feed forward signal. Particularly, parameters of the filter can be set according to a simulation of the behaviour of the electric machine and/or the electromechanical system. Further, it is also possible that parameters of the filter can be varied adaptively during operation of the electric machine. Finally, it is also possible that one or more parameters of the filter belonging to a first group of parameters can be set according to a simulation of the behaviour of the electric machine and one or more of parameters of the filter belonging to a second group can be varied adaptively during operation of the electric machine.
Generally, “feed forward” is a term describing an element or pathway within a control system which passes a control signal from a source in the control system's external environment to a load in its external environment. A control system which is based on feed forward behaviour responds to its control signal in a pre-defined way without responding to how the load reacts; this is in contrast to a system that is based on feedback, which adjusts the output of the control system by taking into account how it affects the load, and how the load itself may vary unpredictably; the load is considered to belong to the external environment of the system. For a control scheme to be reliable by pure feed forward without feedback the effect of the output of the system on the load should be known which usually means that the behaviour of the load is assumed not to change with time in an unpredicted or unexpected manner. Once the control signal has been sent, it cannot be further adjusted. For corrective adjustment a new control signal must be sent.
Particularly, the feed forward signal can bypass a torque control segment of the control unit, i.e. the feed forward signal of the calculated electromagnetic force can be processed in the filter in parallel and independently of the processing of any torque request in the torque control segment of the control unit. Favourably, limitations e.g. of current or torque which are valid for, and have to be observed by, this torque control segment does not limit the feed forward signal of the calculated electromagnetic force and consequently have no impact on the performance of the method according to the invention. The respective control segments of the control unit can take the form of purely hardware or purely software or a combination of hardware and software. An inherent coupling of the systems of inertia is changed in a way that mitigates the oscillatory behaviour of the resonant system.
According to a favourable method step, a step can be performed further providing the feed forward signal as input to a current control segment in addition to at least one of a requested current derived from (i) a corresponding torque request, (ii) an electrical angular speed of the at least one electric machine, (iii) an actual current of the at least one electric machine. The feed forward signal can be provided to the current control segment in addition to at least one of requested parameters (i) requested current, (ii) electrical angular speed of the at least one electric machine, and (iii) actual current in the at least one electric machine. The feed forward signal of the electromotive force can be treated like a normal input signal to the current control segment of the control unit which sends an input signal, e.g. a pulse-width modulation (PWM) signal or a pulse-amplitude modulation signal or the like, to an inverter for operating the electric machine. The coupling between the current control and the electromechanical system via the electrical angular speed which is one of the main causes for oscillations in the system can be addressed in an appropriate way to diminish the oscillations in the system. The electromotive force can be provided as input to the control unit of the at least one electric machine as feed forward signal of the electromagnetic force, particularly the calculated electromagnetic force. In this case, an additional feedback loop is usually not needed. Besides the fact that such a feedback loop could be too slowly for the oscillation damping process in question, the feedback may also introduce other stability or resonant issues which can be avoided by using the feed forward signal (only).
According to a favourable method step, a step can be performed further comprising the step of forwarding control signals to an inverter coupled to the at least one electric machine. Thus the current applied to the electric machine can be adjusted to reduce or avoid an oscillatory behaviour. Expediently, the phase of the signals can be treated in a phase advancing way, i.e. shifted to a phase value ahead of the phase value of the electromotive force, or in a phase retarding way, i.e. shifted to a phase value behind the phase value of the electromotive force. Favourably, the filter employed for adjusting the phase of the angular speed basically effects the phase of the electromotive force signal. The shifted phase can be either shifted to phase values ahead or shifted to phase values behind, wherein the decision whether to shift the phase to phase values ahead or to phase values behind is advantageously based on the result of an analysis of the eigenvalues of the system. By analysing the eigenvalues, those filter parameters which are effecting the phase shift can be set so that the eigenvalue corresponding to the resonance has a negative real part.
According to a favourable method step, one or more parameters of the filter belonging to a first group of parameters can be set according to a simulation of the behaviour of the electric machine and/or the electromechanical system, and/or one or more parameters of the filter belonging to a second group of parameters can be adaptively varied during operation of the electric machine. Favourably, the behaviour of the system can be optimized during operation in a varying ambient and for varying operation conditions. The filter can be optimized to a broad operation range of the electric machine. When the filter parameters can be set according to the simulation of the behaviour of the electric machine and/or the electromechanical system, the filter can shift the phase of the signal by a predefined value and in a predefined direction. This method step is cost efficient and requires only minimum effort during operation of the electric machine.
According to another aspect of the invention, an oscillation damping system is proposed for employing a method for damping electromechanical oscillations, comprising a filter for processing at least one of the steps of (i) deriving actual values of the electromotive force and (ii) adjusting the phase of the angular speed, based on the derived actual values of the electromotive force, in a direction in which an oscillatory behaviour of said electric machine is reduced. Particularly, the filter can be connected to an input of a current control segment of a control unit.
According to a favourable embodiment, the filter can be connected in parallel to a torque control segment which provides an input signal for the current control segment. Favourably, the filter influences the phase of the electrical angular speed fed into the filter which outputs the electromotive force as a feed forward signal to a control unit controlling the electric machine.
According to a favourable embodiment, the filter can be connected to an input of a current control segment of the control unit. Particularly, the filter can bypass a torque control segment of the control unit which torque control segment provides an input signal to the current control segment corresponding to a requested current which is required to generate a requested torque of the electric machine.
According to a favourable embodiment, the filter can be arranged in parallel to a torque control segment. The torque control segment provides an input signal for the current control segment. By bypassing the torque control segment the torque and/or current limitations of the torque control segment do not limit the feed forward signal.
According to a favourable embodiment, the electric machine can be coupled to one or more torque loads characterized by a mechanical angular speed via a spring-damper system.
According to another aspect of the invention, a computer program is proposed comprising a computer program code adapted to perform a method or for use in a method according to at least one of the method features described above when said program is run on a programmable microcomputer. Particularly, the filter can be realized in hardware and/or in software. Preferably, the computer program can be adapted to be downloaded to a control unit or one of its components when run on a computer which is connected to the internet.
According to another aspect of the invention, a computer program product stored on a computer readable medium is proposed, comprising a program code for use in a method according to the invention on a computer.